U.S. patent application number 11/817522 was filed with the patent office on 2009-09-17 for method for soldering electronic component and soldering structure of electronic component.
This patent application is currently assigned to MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.. Invention is credited to Tadashi Maeda, Mitsuru Ozono, Tadahiko Sakai.
Application Number | 20090233117 11/817522 |
Document ID | / |
Family ID | 37768761 |
Filed Date | 2009-09-17 |
United States Patent
Application |
20090233117 |
Kind Code |
A1 |
Sakai; Tadahiko ; et
al. |
September 17, 2009 |
METHOD FOR SOLDERING ELECTRONIC COMPONENT AND SOLDERING STRUCTURE
OF ELECTRONIC COMPONENT
Abstract
In soldering an electronic component, for the purpose of leading
molten solder during re-flow, metallic powder 8 is mixed into flux
employed so as to intervene between a bump and an electrode. The
metallic powder 8 has a flake or dendrite shape including a core
segment 8a of the metal molten at a higher temperature than the
liquid phase temperature of solder constituting a solder bump and a
surface segment 8b of the metal with good-wettability for the
molten solder and to be solid-solved in the core segment 8a molten.
In the heating by the re-flow, the metallic powder remaining in the
flux without being taken in a solder portion is molten and
solidified to become substantially spherical metallic particles 18.
Thus, after the re-flow, the metallic powder does not remain in a
flux residue in a state where migration is likely to occur, thereby
combining both solder connectivity and insurance of insulation.
Inventors: |
Sakai; Tadahiko; (Fukuoka,
JP) ; Maeda; Tadashi; (Fukuoka, JP) ; Ozono;
Mitsuru; (Fukuoka, JP) |
Correspondence
Address: |
PEARNE & GORDON LLP
1801 EAST 9TH STREET, SUITE 1200
CLEVELAND
OH
44114-3108
US
|
Assignee: |
MATSUSHITA ELECTRIC INDUSTRIAL CO.,
LTD.
Osaka
JP
|
Family ID: |
37768761 |
Appl. No.: |
11/817522 |
Filed: |
November 10, 2006 |
PCT Filed: |
November 10, 2006 |
PCT NO: |
PCT/JP2006/322902 |
371 Date: |
August 31, 2007 |
Current U.S.
Class: |
428/551 ;
228/205; 228/256 |
Current CPC
Class: |
H05K 3/3489 20130101;
H05K 2203/0435 20130101; H05K 2201/0215 20130101; H01L 2224/16225
20130101; Y10T 428/12049 20150115; H05K 2201/10977 20130101; H05K
2201/0245 20130101; H05K 3/3436 20130101; H05K 2201/0218 20130101;
H01L 2224/05573 20130101; H05K 3/3485 20200801; H01L 24/81
20130101; H05K 2201/0248 20130101 |
Class at
Publication: |
428/551 ;
228/256; 228/205 |
International
Class: |
B32B 15/04 20060101
B32B015/04; B23K 31/02 20060101 B23K031/02; B23K 1/20 20060101
B23K001/20 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 10, 2005 |
JP |
2005-325707 |
Claims
1. A method for soldering an electronic component on a substrate
such that a solder bump formed on the electronic component aligned
with an electrode of the substrate is heated and the solder bump
thus molten is soldered on the electrode, comprising the steps of:
aligning the solder bump of said electronic component with the
electrode of the substrate with flux intervening between the solder
bump and the electrode, the flux containing metallic powder having
a flake or dendrite shape including a core segment of the metal
molten at a higher temperature than the liquid phase temperature of
solder constituting said solder bump and a surface segment of the
metal with good-wettability for said solder molten and to be
solid-solved in said core segment molten; heating said electronic
component and said substrate to melt said solder bump so that the
molten solder is wet and spread along the surface of the metallic
powder to reach the electrode; further continuing heating so that
the metallic powder remaining without being in contact with said
molten solder is molten into substantially spherical shape; and
thereafter cooling said substrate and said electronic component so
that the molten metallic powder and said solder is solidified.
2. A method for soldering an electronic component according to
claim 1, wherein the metal constituting said surface segment is
gold (Au), silver (Ag) or platinum (Pt); and the metal constituting
said core segment is tin (Sn) or a tin-series alloy.
3. A method for soldering an electronic component according to
claim 1, further comprising the step of cleaning the metallic
powder solidified to become spherical and residue of said flux
using a cleaning water so that they are removed from the
substrate.
4. A method for soldering an electronic component on a substrate
such that a solder bump formed on the electronic component aligned
with an electrode of the substrate is heated and the solder bump
thus molten is soldered on the electrode, comprising the steps of:
aligning the solder bump of said electronic component with the
electrode of the substrate with thermosetting resin intervening
between the solder bump and the electrode, the thermosetting resin
containing metallic powder having a flake or dendrite shape
including a core segment of the metal molten at a higher
temperature than the liquid phase temperature of solder
constituting said solder bump and a surface segment of the metal
with good-wettability for said solder molten and to be solid-solved
in said core segment molten; heating said electronic component and
said substrate to melt said solder bump so that the molten solder
is wet and spread along the surface of the metallic powder to reach
the electrode; further continuing the heating so that the metallic
powder remaining without being in contact with said molten solder
is molten into substantially spherical shape; promoting a hardening
reaction of said thermosetting resin by said heating; and
thereafter cooling said substrate and said electronic component so
that the molten metallic powder and said solder are solidified.
5. A method for soldering an electronic component according to
claim 4, wherein the metal constituting said surface segment is
gold (Au), silver (Ag) or platinum (Pt); and the metal constituting
said core segment is tin (Sn) or a tin-series alloy.
6. A soldering structure of an electronic component soldered on a
substrate by the soldering method according to claim 1, comprising
having a solder portion for connecting said electronic component
and said electrode and a flux residue remaining on the surfaces of
said solder portion and of the substrate, wherein metallic
particles creates as a result that the metallic powder not brought
into contact with the molten solder is molten to become spherical
are contained in said flux residue.
7. A soldering structure of an electronic component according to
claim 6, wherein said metallic particles each has an oxidation film
on the surface.
8. A soldering structure of an electronic component soldered on a
substrate by the soldering method according to claim 4, comprising
having a solder portion for connecting said bump and said electrode
of the substrate and a resin portion for reinforcing a connecting
portion between the solder portion and the electrode, wherein
metallic powder created as a result that said metallic powder not
brought into contact with the molten solder is molten to become
substantially spherical are contained in said resin portion.
9. A soldering structure of an electronic component according to
claim 8, wherein said metallic particles each has an oxidation film
on the surface.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates to a method for soldering an
electronic component on a substrate and a soldering structure of
the electronic component.
[0003] 2. Related Art
[0004] As a method for mounting an electronic component on a
substrate, traditionally, soldering has been widely employed. The
manner of soldering includes various techniques such as a method of
forming a metallic bump serving as a connecting electrode formed on
the electronic component by soldering and a method of
"solder-precoating" of forming a solder layer on the electrode
surface of the substrate. In recent years, from the point of view
of environmental protection, in the above soldering, "lead-free
solder" containing a least quantity of harmful lead has been
adopted.
[0005] The lead-free solder is greatly different from the
lead-series solder conventionally employed in their composition.
Therefore, as regards the flux employed in a soldering process, the
flux conventionally generally employed cannot be adopted as it is.
Specifically, the conventional flux has an insufficient activating
operation so that removal of an oxidation film on a solder surface
is insufficient. Thus, it is difficult to assure good
solder-wettability. For such solder having inferior
solder-wettability, the flux having the composition in which
metallic power of the metal with excellent solder-wettability such
as silver is mixed has been proposed (for example, see
JP-A-2000-31210). By using such flux, the solder molten in a
re-flow process can be wet-spread along the surface of the metallic
powder in the flux so that the solder molten can be led to an
electrode which is a connecting target.
[0006] However, in the flux disclosed in the above Patent
Reference, as the case may be, the following inconvenience has
occurred according to the contents of the metallic powder. The
mainstream in recent years is a non-cleaning technique in which
cleaning for eliminating the flux component is not carried out
after the soldering. Therefore, after the re-flow, the flux
component remains as a residue deposited around a soldered portion.
So the metallic powder contained in the flux also remains around
the soldered portion.
[0007] At this time, if a large quantity of metallic powder
remains, poor insulation due to migration may occur. In this case,
if the contents of the metallic powder are reduced in order to
prevent the poor insulation, the effect of leading the molten
solder by the metallic powder in the re-flow process is lowered. As
a result, the solder-connectivity was deteriorated. Particularly,
in the soldering for manufacturing a semiconductor device by
stacking a package component with semiconductor elements packaged
in a resin substrate, a gap is likely to occur between the bump to
be soldered and an electrode owing to warping of the resin
substrate. As a result, poor connection due to solder-wettability
occurred with high frequency. As described above, the conventional
soldering method using the flux containing the metallic powder has
a problem that it is difficult to combine the maintenance of
solder-connectivity and assurance of insulation.
SUMMARY OF THE INVENTION
[0008] In view of the above circumstances, an object of this
invention is to provide a method of soldering an electronic
component, capable of combining solder-connectivity and assurance
of insulation and a soldering structure of the electronic
component.
[0009] One aspect of the invention provides a method for soldering
an electronic component on a substrate such that a solder bump
formed on the electronic component aligned with an electrode of the
substrate is heated and the solder bump thus molten is soldered on
the electrode, comprising the steps of: aligning the solder bump of
the electronic component with the electrode of the substrate with
flux intervening between the solder bump and the electrode, the
flux containing metallic powder having a flake or dendrite shape
including a core segment of the metal molten at a higher
temperature than the liquid phase temperature of solder
constituting the solder bump and a surface segment of the metal
with good-wettability for the solder molten and to be solid-solved
in the core segment molten; heating the electronic component and
the substrate to melt the solder bump so that the molten solder is
wet and spread along the surface of the metallic powder to reach
the electrode; further continuing heating so that the metallic
powder remaining without being in contact with the molten solder is
molten into substantially spherical shape; and thereafter cooling
the substrate and the electronic component so that the molten
metallic powder and the solder are solidified.
[0010] Another aspect of the invention provides a method for
soldering an electronic component on a substrate such that a solder
bump formed on the electronic component aligned with an electrode
of the substrate is heated and the solder bump thus molten is
soldered on the electrode, comprising the steps of: aligning the
solder bump of the electronic component with the electrode of the
substrate with thermosetting resin intervening between the solder
bump and the electrode, the thermosetting resin containing metallic
powder having a flake or dendrite shape including a core segment of
the metal molten at a higher temperature than the liquid phase
temperature of solder constituting the solder bump and a surface
segment of the metal with good-wettability for the solder molten
and to be solid-solved in the core segment molten; heating the
electronic component and the substrate to melt the solder bump so
that the molten solder is wet and spread along the surface of the
metallic powder to reach the electrode; further continuing the
heating so that the metallic powder remaining without being in
contact with the molten solder is molten into substantially
spherical shape; promoting a hardening reaction of the
thermosetting resin by the heating; and thereafter cooling the
substrate and the electronic component so that the molten metallic
powder and the solder are solidified.
[0011] Another aspect of the invention provides a soldering
structure of an electronic component soldered on a substrate by the
soldering method, comprising having a solder portion for connecting
the electronic component and the electrode and a flux residue
remaining on the surfaces of the solder portion and of the
substrate, wherein metallic particles created as a result that the
metallic powder not brought into contact with the molten solder is
molten to become spherical are contained in the flux residue.
[0012] Another aspect of the invention provides a soldering
structure of an electronic component soldered on a substrate by the
soldering method, comprising having a solder portion for connecting
the bump and the electrode of the substrate and a resin portion for
reinforcing a connecting portion between the solder portion and the
electrode, wherein metallic powder created as a result that the
metallic powder not brought into contact with the molten solder are
molten to become substantially spherical are contained in the resin
portion.
[0013] According to the invention, the metallic powder mixed for
the purpose of leading molten solder during re-flow has a flake or
dendrite shape including a core segment of the metal molten at a
higher temperature than the liquid phase temperature of solder
constituting a solder bump and a surface segment of the metal with
good-wettability for the molten solder and to be solid-solved in
the core segment molten.
[0014] Therefore, the remaining metallic powder is molten during
the re-flow to fall in their substantially spherical state which is
difficult to generate migration. Thus, both solder connectivity and
insurance of insulation can be combined.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIGS. 1A to 1C are views for explaining the process of
mounting an electronic component according to the first embodiment
of this invention.
[0016] FIGS. 2A to 2C are views for explaining the process of
mounting an electronic component according to the first embodiment
of this invention.
[0017] FIGS. 3A and 3B are views for explaining the shape and
composition of the flux employed in the method for soldering an
electronic component according to the first embodiment of this
invention.
[0018] FIGS. 4A to 4C are views for explaining the process of
solder-connection in the method for soldering an electronic
component according to the first embodiment.
[0019] FIGS. 5A to 5C are sectional views of a metallic powder
contained in the flux employed in the method for soldering an
electronic component according to the first embodiment of this
invention.
[0020] FIG. 6 is a sectional view of the soldering structure of an
electronic component according to the first embodiment.
[0021] FIGS. 7A to 7C are views for explaining the method for
supplying paste for solder-connection in the mounting of an
electronic component according to the first embodiment of this
invention.
[0022] FIG. 8 is a sectional view of the soldering structure of an
electronic component according to the second embodiment of this
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Embodiment 1
[0023] FIGS. 1A to 1C and 2A to 2C are views for explaining the
process of a method for soldering an electronic component according
to the first embodiment of this invention. FIGS. 3A and 3B are
views for explaining the shape and composition of metallic powder
contained in the flux employed in the method for soldering an
electronic component according to the first embodiment of this
invention. FIGS. 4A to 4C are views for explaining the process of
solder-connection in the method for soldering an electronic
component according to the first embodiment of this invention.
FIGS. 5A to 5C are sectional views of the metallic powder contained
in the flux employed in the method for soldering an electronic
component according to the first embodiment of this invention. FIG.
6 is a sectional view of a soldering structure of the electronic
component in the first embodiment of this invention. FIGS. 7A to 7C
are views for explaining the method for supplying paste for
solder-connection in mounting an electronic component according to
the first embodiment of this invention.
[0024] First, referring to FIGS. 1A to 1C and 2A to 2C, an
explanation will be given of a method for soldering an electronic
component. In this method for soldering an electronic component, a
solder bump formed on the electronic component aligned with an
electrode of a substrate is heated and the solder bump thus molten
is solder-connected to the electrode. By such a soldering method,
the electronic component is mounted on the substrate.
[0025] In FIG. 1A, electrodes 2 are formed on the upper surface of
a substrate 1. An electronic component 4 has a structure in which
component electrodes 4b are formed on the lower surface of a resin
substrate 4a with a component mounting portion 5 formed on the
upper surface, and solder bumps 6 (hereinafter simply referred to
as bumps 6) are formed on the component electrodes 4b. The bumps 6
are formed by solder-connecting a fine-granular solder ball to the
component electrodes 4b. Incidentally, the "solder" refers to a
metal having a low melting point (e.g. tin) or alloy composed of
plural kinds of metals (e.g. silver-tin alloy). Now, the lead-free
solder containing a least quantity of lead in these metal and alloy
is employed as a solder material.
[0026] The component mounting portion 5 is formed by resin-sealing
a semiconductor element (not shown) mounted on the upper surface of
the resin substrate 4a. In this resin-sealing step, the resin at a
high temperature in a molten state is injected into a mold cavity
and thermally hardened to form a resin mold. The resin mold thus
formed is removed from the mold cavity and cooled in the air. In
this cooling step, owing to a difference in a thermal expansion
coefficient between the resin substrate 4a and the resin mold, the
component mounting portion 5 on the upper side of the resin
substrate 4a contracts more greatly than the resin substrate 4a.
Thus, the entire electronic component 4 is deformed in such a
fashion that the end of the resin substrate 4a is warped toward the
component mounting portion 5.
[0027] Thus, among the plurality of bumps 6 formed on the lower
side of the electronic component 4, the lower end of each of the
bumps 6* located at the outer edges is located at a higher position
by a displacement d1 due to warping deformation than that of each
of the bumps located inside. Therefore, the heights of the lower
ends of the bumps 6 are not in flush with one another so that in a
status where the electronic component 4 is loaded on the substrate
1, a gap is likely to occur between the bumps 6* and the
corresponding electrodes 2.
[0028] Flux 3 as described below is applied on each of the bumps 6
by duplication. Specifically, by moving up and down the electronic
component 4 for a duplicating table 7 with a deposited film of the
flux 3, as seen from FIG. 1B, the flux 3 is duplicated/applied on
the lower end of each of the bumps 6. The flux 3, in the
solder-connection for soldering the electronic component 4 on the
substrate 1 described below, is employed to intervene between bumps
6 and electrodes 2 in order to improve the solder-connectivity.
[0029] Now, the composition of the flux 3 will be explained. The
flux 3 is obtained by mixing an activator and metallic powder 8 as
additives into a liquid base agent with high viscosity in which a
resin component such as rosin is solid-solved in a solvent. The
activator is added for the purpose or removing an oxidation film of
the solder created on the surface of each the bumps 6. To this end,
an organic acid having a capability of removing the oxidation film
is employed. Incidentally, now, as the activator, the activator
having low activity which is not required to be cleaned after the
solder connection is adopted.
[0030] As the metallic powder 8, as seen from FIG. 3A, a flake
powder formed by crushing a fine-granular metal is employed. FIG.
3B shows a section taken along line A-A of the metallic powder 8 in
FIG. 3A. As seen from FIG. 3B, the metallic powder 8 is composed of
a core segment 8a and a surface segment 8b covering the surface of
the core segment 8a. At the boundary between the surface segment 8b
and the core segment 8a, a diffused layer 8c in which the metal
constituting the surface segment 8b is diffused in the core segment
8a is formed. In such a configuration, the species of metal used in
the core segment 8a is selected from the group consisting of tin
(Sn) and tin-series alloy. The surface segment 8b is formed by
covering the surface of the core segment 8a by the technique such
as electroplating.
[0031] The tin series alloy may be a tin-silver (Sn--Ag) series,
tin-silver-copper (Sn--Ag--Cu) series, tin-lead (Sn--Pb) series,
tin-lead-silver (Sn--Pb--Ag) series, tin-copper (Sn--Cu) series,
tin-bismuth (Sn--Bi) series, tin-silver-bismuth (Sn--Ag--Bi)
series, tin-silver-bismuth-indium (Sn--Ag--Bi--In) series,
tin-antimony (Sn--Sb) series, tin-indium (Sn--In) series, tin-zinc
(Sn--Zn) series, tin-zinc-bismuth (Sn--Zn--Bi) series, or
tin-zinc-aluminum (Sn--Zn--Al) series.
[0032] As a metallic species of the surface segment 8b, selected is
the material which will be molten at a higher temperature than the
liquid phase temperature of the solder employed for the bump 6 and
does not create the oxidation film on the surface of the metallic
powder 8 in the air and further gives good wettability for the
solder constituting the bump 6 so that the solder in the flowing
state of the molten bump 6 is likely to be wet and spread along the
surface (e.g. noble metal such as gold (Au), silver (Ag) or
platinum (Pt) having purity of 90% or more). Its addition to the
flux 3 is executed by mixing such metallic powder 8 into the basic
agent with a percent within a range of 1 to 20 vol %.
[0033] Now, as the combination of the metal species employed in the
core segment 8a and the surface segment 8b, selected is a
combination capable of realizing a diffusion characteristic that
the diffusion from the surface segment 8b to the internal core
segment 8a (see FIG. 5B is likely to occur by heating in a re-flow
process and when the re-flow is completed, the diffusion from the
surface segment 8b to the core segment 8a is completed so that the
greater part of metal in the surface segment 8b is taken into the
core segment 8a. In such a composition, the surface segment 8b is
made of the metal with excellent wettability for the solder, and
the core segment 8a is made of the metal permitting the surface
segment 8b to be molten by heating due to re-flow and internally
taken. By adopting such a composition for the metallic powder to be
mixed into the flux 3, the excellent effect described later can be
obtained in the solder-connection by the non-cleaning system.
[0034] In the above embodiment, the metallic powder 8 is flake so
that the surface area per unit weight is as possible as large.
However, the metallic powder 8 may be dendrite so that fine
bar-like metal is branched three-dimensionally. Such branched
metallic powder can also increase the surface area per unit weight
to the utmost.
If the flake metallic powder and dendrite powder are mixed with
each other, the surface are per unit weight can be increased. In
addition, the structural features of both types of powders are
combined in a status where the metallic powder is mixed into the
flux 3 so that the metallic powder by a small weight % can be
distributed uniformly and with high density to the utmost in the
flux 3.
[0035] Namely, the flux 3 adopted in the soldering method according
to this embodiment contains the flake or dendrite metallic powder 8
composed of the core segment 8a of the metal which is molten at a
higher temperature than the liquid-phase of the solder constituting
the solder bump 6 and the surface segment 8b of the metal which has
good wettability for the molten solder and is solid-solved into the
molten core segment 8a.
[0036] Next, as seen from FIG. 1C, the electronic component 4 after
the flux has been duplicated/applied is mounted on the substrate 1.
The mounting of the electronic component 4 on the substrate 1 is
carried out by melting the bumps 6 by heating so that the molten
bumps 6 are continuously solder-connected to the upper surface of
the electrodes. Thus, the respective component electrodes 4b are
electrically connected to the corresponding electrodes 2 and the
electronic component 4 is fixed to the substrate 1 through the
solder portions which have been formed by solidification of the
molten solder.
[0037] In this mounting process, with the bumps 6 being aligned
with the electrodes 2, the electronic component 4 located above the
substrate 1 is lowered toward the substrate 1. The bumps 6 with the
flux 3 applied are landed on the electrodes 2 and pressed by a
predetermined pressing load. Thus, of the plurality of bumps 6,
even if the bumps 6 with the lower end located at an average height
have slight changes in their height, their lower ends are brought
into contact with the upper surface of the electrodes 2 because the
higher bumps 6 are slightly crushed by the pressing force in the
height direction. On the other hand, even when the other bumps 6
are slightly crushed so that the entire electronic component 4 is
lowered correspondingly, their lower ends of the bumps 6* located
at the outer edges are not brought into contact with the surface of
the electrodes 2. Thus, the bumps 6* are in a state where there is
a gap between the lower surface of the bumps and the electrodes
2.
[0038] Next, an explanation will be given of the step of melting
the bumps 6 so as to be solder-connected to the electrodes 2. The
substrate 1 with the electronic component loaded as shown in FIG.
1C is forwarded to a re-flow furnace and heated. At this time as
shown in FIG. 2A, heating is carried out in a status where in the
bumps 6 in the vicinity of the center whose lower ends are located
at the average height, their lower ends are kept in contact with
the electrodes 2 and in the bumps 6* located at the outer edges,
the flux 3 intervene between their lower ends and the electrodes
2.
[0039] By this heating, both bumps 6, 6* are solder-connected to
the electrodes 2. The behavior of the solder at this time differs
according to whether or not the bump lower end is in contact with
the electrode 2. Specifically, as seen from FIG. 2B, in the bump 6
with the lower end being in contact with the electrode 2, when it
is molten by heating, the solder 6a in the molten state immediately
spreads favorably along the surface of the electrode 2 of the
material with good solder-wettability so that the component
electrode 4b is coupled through the electrode 2 and the solder 6a.
At this time, the oxidation film on a surface of the bump 6 is
removed by the activator contained in the flux 3.
[0040] On the other hand, in the bump 6* with the flux 3 residing
in the gap between itself and the electrode 2, the component
electrode 4b and the electrode 2 are coupled with each other by the
solder 6a via the process shown in FIGS. 4A to 4C. FIG. 4A shows
the state when heating is started in the re-flow process.
[0041] Now, the metallic powder 8 in the flux 3 intervening the
lower end of the bump 6* and the surface 2a of the electrode 2
remains mixed in the flux 3, i.e. reside in a state the flake core
segment 8a is covered with the surface segment 8b as shown in FIG.
5A. Since a lot of the metallic powder 8 resides in a random
posture within the flux 3, bridges of the metallic powder 8
coupling the lower end of the bump 6* and the surface 2a of the
electrode 2 are formed with a high probability (see portion
indicated by arrow a in FIG. 4A).
[0042] Now, the "bridge" refers to the state where the metallic
powder 8 resides as a continuous series in a state adjacent to each
other. The "adjacent state" refers to the state where a plurality
of metallic powders 8 reside at intervals such that when the solder
in a flowing state wet-covering the surface of a certain metallic
powder 8 creates a certain thickness by its surface tension, the
surface of the solder thickness is brought into contact with the
contiguous other metallic powder 8.
[0043] Specifically, since the large amount of metallic powder 8
resides continuously in such a contiguous state, the solder in
contact with the metallic powder 8 on the one side of the series of
the metallic powder is wet and spreads so as to wrap the surface of
the metallic powder 8 containing the metal with good
solder-wettability and so it is successively brought into contact
with the adjacent metallic powders 8. The flow of the solder
through this propagation is continuously created to the other side
of the series of the metallic powder. Thus, the series of the
metallic powder 8, as seen from FIG. 4B, serve as a bridge which
couples the lower end of the bump 6* with the surface 2a of the
electrode 2 so that the solder flows.
[0044] In this case, since the surface segment 8b constituting the
metallic powder 8 is made of the noble metal such as gold or silver
having a higher melting point than the liquid phase temperature of
the solder generally employed, even if it is heated to the higher
temperature than the liquid phase temperature of the solder, the
surface segment 8b surely resides in a solid state. Specifically,
in the solder-connecting method using creamy solder containing
solder particles in the flux 3, the solder particles in the creamy
solder are simultaneously molten due to heating in the re-flow so
that the function of bridging the molten solder within the gap
cannot be obtained. On the other hand, in the flux 3 according to
this embodiment, the metallic powder can surely carry out the above
bridging function.
[0045] The metallic power 8 employed in the flux 3 uses expensive
noble metal such as gold or silver as the surface segment 8b
covering the surface of the inexpensive core segment 8a. For this
reason, as compared with the method of using the expensive noble
metal, as it is, as powder body in the conventional metallic-powder
containing flux, great cost reduction can be realized.
Incidentally, although the solder made of the alloy (e.g. Sn--Ag
series solder) composed of the metal species selectable as the core
segment 8a and silver has already been proposed, such solder should
be definitely distinguished from the metallic powder 8 according to
this embodiment from the point of view of the operational advantage
obtained by the metallic powder 8.
[0046] Now, by using the flake shape of the metallic powder 8
obtained by processing the above metal, the bridges can be easily
formed by the metallic powder 8 residing with the posture with its
longitudinal direction oriented in the gap bridging direction.
Thus, the bridges can be formed effectively with the low percentage
of contents. Once the solder 6a reaches the electrode surface 2a
via such a bridge, the solder 6a in the flowing state wet-expands
along the electrode surface 2a with good solder-wettability. Owing
to the wet-expansion of the solder 6a, the flux 3 in the vicinity
of the electrode surface 2a is pushed aside so that in also the
bump 6* having initially giver the gap between itself and the
electrode 2, the component electrode 4b is completely coupled with
the electrode 2 by the solder 6a.
[0047] In this case also, the connectivity is improved by the
activator contained in the flux 3. However, owing to the above
bridge forming effect, even where the oxidation film on the bump
surface is only partially removed, the activator contained in the
flux 3 is not required to have a strong activating action. In other
words, addition of the metallic powder 8 permits the low-activity
flux with weak activating action to be employed. Thus, even where
the flux 3 remains after solder-connection, the degree of corrosion
of the electrode 2 by the active component is low. Accordingly, in
cooperation with the effect of improving the insulation due to the
characteristic of the metallic powder 8 described later, also in
the non-cleaning technique in which cleaning for removing the flux
is not carried out after the solder-connection, sufficient
reliability can be assured.
[0048] In the above re-flow process, in the metallic powder 8 of a
piece as shown in FIG. 5A, by continuing the heating, as shown in
FIG. 5B, the surface segment 8b is gradually taken into the core
segment 8a by diffusion. According to the metal specie of the core
segment 8a and the heating temperature, there are two cases where
the surface segment 8b is diffused into the core segment 8a in a
liquid phase and where the surface segment 8b is diffused into the
core segment 8a In a solid phase. In both cases, the surface
segment 8b is gradually taken in the core segment 8a.
[0049] Since the metallic powder 8 is heated to the temperature
higher than the melting point of the metal constituting the core
segment 8a, the core segment 8a with the surface segment 8b taken
in by solid-solution is molten and the core segment 8a thus molten
is condensed by surface tension. The core segment 8a is
subsequently cooled and solidified, which results in a metallic
particle 18 having a substantially spherical shape, as shown in
FIG. 5C. Namely, by taking the surface segment 8b into the core
segment 8a by solid-solution, the metallic powder 8 containing the
noble metal such as silver having a high melting point as the
surface segment 8b can be shape-changed into the metallic particle
18 by the heating during the re-flow process. By exposure of the
surface of the core segment 8a with the surface segment 8b
completely taken in as well as the above shape-change, an oxidation
film 8d due to heating and oxidation of the core segment 8a is
formed on the surface of the metallic particle 18. The oxidation
film 8d presents an effect of improving the insulation after
solder-connection as described later.
[0050] FIG. 4C slows the state where the solder 6a and the metallic
particle 18 are solidified by cooling the electronic component 4
and substrate 1 after a predetermined heating cycle during the
re-low process has been completed. By solidification of the solder
6a, a solder portion 16 connecting the component electrode 4b and
the electrode 2 by soldering is formed. In the vicinity of the
electrode surface 2a of the solder portion 16, the metallic powder
8 taken in the solder during the soldering process resides in an
alloy state or solid-solution state. On the electrode surface 2a
and around the electrode 2, the residue (resin component and
activator) 3a after the solvent component has been evaporated from
the flux 3 remains together with the substantially spherical
metallic particles 18 generated by melting and solidification of
the metallic powder 8 not taken into the solder portion 16.
[0051] FIG. 2C shows the state where soldering of the electronic
component 4b on the substrate 1 has been completed as a result that
the solder portions 16 each coupling the component electrode 4 with
the electrode 2 were formed for all the component electrodes 4b and
electrodes 2. Thus, the soldering structure of the electronic
component as shown in FIG. 6 is manufactured. As seen from FIG. 6,
the component electrode 4b and the electrode 2 are connected by the
solder portion 16, and around the electrode 2, the flux residue 3a
partially covering the lower part of the solder portion 16 and
extended to the surface of the substrate 1 remains in a deposited
state. In the flux residue 3a, the metallic particles 18 are
scattered which remain as a result that they have not been brought
into contact with the molten solder 6a during the re-flow process
and not taken into the solder portion 16.
[0052] In short, the soldering structure of the electronic
component is in a configuration having the solder portions 16 each
connecting the component electrode 4b of the electronic component 4
to the electrode 2 and flux residue 3a remaining on the surfaces of
the solder portions 16 and of the substrate 1, in which the
metallic powder 8 not brought into contact with the molten solders
6a is molten to form a substantially spherical shape and the
metallic particles 18 each having the oxidation film 8d on the
surface are contained in the flux residue 3a. Such a soldering
structure can present the following excellent effects in ensuring
the insulation between the electrodes.
[0053] In the non-cleaning method in which cleaning for removing
the flux is not carried out after the solder-connection step, the
flux residue 3a remains around the electrodes 2 as it is. Where the
metal such as gold or silver is employed as the metallic powder to
be mixed into the flux as it is, according to the quantity of the
residue, migration may occur which electrically corrodes the area
between the electrodes thereby to deteriorate the insulation
therebetween. Therefore, traditionally, in view of assuring the
insulation, it was necessary to restrict the metallic powder to be
mixed to a low percentage of contents. As a result, a situation was
occurred that the effect of improving the solder-wettability
leading the molten solder in the re-flow process is not
sufficiently realized.
[0054] On the other hand, using the metallic powder 8 having the
composition described above, even where a considerable amount of
the metallic powder 8 remains around the electrodes 2 after the
solder-connection step has been completed, the metallic powder 8 is
molten by the heating during the re-flow to become the
substantially spherical metallic particle 18. Therefore, the
probability generating the state where the particles are in contact
with one another to be coupled is very low. Further, in cooperation
with the fact that the surface of the metallic particle 18 is
covered with the oxidation film 8d which is electrically stable,
the occurrence of migration can be effectively prevented, thereby
ensuring good insulation. Thus, by using the metallic powder 8
having the composition described above such that the metallic
powder of a quantity enough to assure the solder-wettability in the
flux is mixed, the solder-connectivity can be improved and the
insulation after the solder-connection can be also ensured, thereby
improving the reliability of mounting.
[0055] In other words, by using the metallic powder 8 having the
composition described above, the flux 3 of the non-cleaning type
excellent in both solder-connectivity and insulation can be
realized. Specifically, in the case where the electronic component
with bumps formed of lead-free solder which is high in the hardness
and difficult to be crushed is a target of mounting, also in a
state where there are gaps between the bumps and the circuit
electrodes of the substrate owing to warping of the electronic
component and changes in the bump size, it is possible to
effectively prevent occurrence of the poor mounting that the bump
is not normally soldered on the circuit electrode. In addition,
also where the non-cleaning method not carrying Cut the cleaning
for flux removal after the soldering is adopted, satisfactory
insulation can be ensured.
[0056] Additionally, in the above embodiment, the non-cleaning
method not carrying out the cleaning for flux removal after the
re-flow was adopted. However, where higher reliability is required,
the metallic particles 18 solidified to become spherical and the
flux residue 3a are cleaned by cleaning water so that they are
removed from the substrate 1. In this cleaning step, since the
remaining metallic powder resides in the flux residue 3a in the
form of the metallic particles 18 which can be easily removed, good
cleaning quality can ensured by a simple cleaning method.
[0057] The above method for soldering an electronic component is in
a configuration in which the solder bump 6 is aligned with the
electrode 2 with the flux 3 having the above composition
intervening between the solder bump 6 and the electrode 2, the
electronic component 4 and the substrate 1 are heated to melt the
solder bump 6, the molten solder is wet and spread along the
surface of the metallic powder 8 to reach the electrode 2, heating
is further continued so that the metallic powder 8 remaining
without being in contact with molten solder is molten into
substantially spherical shape, and thereafter the substrate 1 and
the electronic component 4 are cooled so that the molten metallic
powder 8 and the solder are solidified.
[0058] In the embodiment described above, in a step of applying the
flux 3, the flux 3 is duplicated on the bumps 6. However, other
various methods can be adopted. For example, as seen from FIG. 7A,
the flux 3 is jetted by a dispenser 9 so that it is supplied to the
electrode 2. Further, as seen from FIG. 7B, using a duplicating pin
10, the flux 3 may be supplied to the electrode 2 by
duplication.
[0059] Further, as seen from FIG. 7C, by screen printing, the flux
3 may be printed on the electrodes 2. Specifically, a mask plate 11
with pattern holes 11a corresponding to the electrodes 2 is mounted
on the substrate 1 and the flux 3 filled in the pattern holes 11a
by a squeeze 12 is printed on the surface of the electrodes 2.
Embodiment 2
[0060] FIG. 8 is a sectional view of a soldering structure of an
electronic component according to the second embodiment of this
invention. In the second embodiment, as a solder-connection
assistant for enhancing the solder-connectivity between the bump 6
and the electrode 2, a thermosetting resin having an activating
function is employed in place of the flux 3 when the electronic
component 3 as in the first embodiment is soldered to the substrate
1.
[0061] As seen from FIG. 8, as in the first embodiment, the
component electrodes 4b and the electrodes 2 are connected by the
solder portions 16, respectively. Around the electrodes 2, resin
portions 13 each having a shape partially covering the lower part
of the solder portions 16 and expanded to the surface of the
substrate 1 are formed. The resin portion 13 is formed when the
thermosetting resin used as the solder-connection assistant is
thermally hardened by the heating during the re-flow process and
has a function of reinforcing the connecting portion between the
solder portion 16 and the electrode 2. In the resin portion 13, the
metallic particles 18 are scattered which remain as a result that
they have not been brought into contact with the solder 6a molten
during the re-flow process and not taken into the solder portion
16.
[0062] The soldering structure of an electronic component is in a
configuration having solder portions 16 each for connecting the
component electrode 4b of the electronic component 4 and electrode
2 and resin portions 13 each for reinforcing the connecting portion
between the solder portion 16 and the electrode 2, wherein the
metallic powder 8 not brought into contact with the molten solder
is molten to become substantially spherical and metallic particles
18 thus created each having the oxidation film 8d on the surface
are contained in the resin portion 13. By such a soldering
structure, like the first embodiment, the insulation between the
electrodes can be assured and the connecting portion between the
solder portion 16 and the electrode 2 can be effectively
reinforced, thereby further improving the reliability of
mounting.
[0063] This soldering structure can be realized by substituting the
thermosetting resin having an activating function for the flux 3 in
the method of soldering an electronic component according to the
first embodiment (see FIGS. 1A to 1C and 2A to 2C). Namely, in the
method for soldering an electronic component according to the
second embodiment is in a configuration in which the bump 6 of the
electronic component 1 is aligned with the electrode 2 of the
substrate 1 with the thermosetting resin containing the same
metallic powder 8 as in the first embodiment intervening between
the bump 6 and the electrode 2, the electronic component 4 and the
substrate 1 are subsequently heated to melt the bump 6, and the
solder thus molten is wet and spread along the surface of the
metallic powder 8 to reach the electrode 2.
[0064] The heating is further continued so that the metallic powder
8 remaining without being in contact with molten solder is molten
into substantially spherical shape, thereby promoting the hardening
reaction of the thermosetting resin. Thereafter, the substrate 1
and the electronic component 4 are cooled so that the molten
metallic powder 8 and the solder 6a is solidified. In this
soldering method, as in the first embodiment, improvement of the
solder-connectivity and insurance of the insulation can be
combined.
INDUSTRIAL APPLICABILITY
[0065] The method for soldering an electronic component and
soldering structure of the electronic component according to this
invention provide the effect of combining improvement of the solder
connectivity and insurance of the insulation. They are useful for
the use of soldering prone to generate a gap between the bump and
the electrode, such as soldering for stacking a package component
to manufacture a semiconductor device.
* * * * *